This invention relates to a hydraulic servo valve suitable for use in dirty and explosive environments.
Hydraulic machinery is often called upon to operate in situations in which it is exposed to dust, dirt and explosive atmospheres, a coal mine for example. In machinery for use in coal mines it is found that the working hydraulic fluid is frequently contaminated with dust which tends to jam electrohydraulic servo valves. To overcome this, the electrical actuators of the servo valves are made to be unusually powerful, so that the spool can displace grit in the seatings. However, such powerful electrical actuation requires that relatively high levels of electrical power must be supplied, which may well be sufficient to initiate an explosion in a "fiery" mine, should any failure of insulation occur. For this reason, it is at present necessary to enclose the electro-hydraulic valves of mining equipment in expensive flame-proof enclosures, which are of sufficient thickness to withstand explosion of the atmosphere within.
Electroviscous valves employing electroviscous fluids are known for use in the control of hydraulic servo valves, as described in UK Patent No. 1511658 for example. Electroviscous fluids undergo a change in shear resistance when they are exposed to electric fields, typically of 2 kV over a 1 mm gap, which property is exploited in an elecroviscous valve, the fluid being passed between electrodes to which suitably variable voltages can be applied so as to vary the fluid flow rate through the valve. Although the voltages required are high, the current requirement is very low and the total electrical power demanded would generally be insufficient to initiate an explosion in an explosive atmosphere. For additional safety, a high impedance source can be employed to eliminate explosion risk in the event of a short circuit. In principle therefore, it is possible to make an electroviscous device inherently safe, so that it does not require a flameproof enclosure. However, in an environment in which contamination of working fluids is likely to occur, electroviscous fluid cannot be used as the primary working fluid, nor as a secondary control fluid in an open circuit, as dirt in the fluid would rapidly lead to failure due to arcing at the electroviscous valve electrodes.
The present invention seeks to provide a hydraulic servo valve controlled by means of an entirely enclosed electroviscous fluid circuit requiring relatively little electrical command power.
In accordance with the present invention a hydraulic servo valve having an adjustable control valve for controlling flow of a pressurised primary fluid includes: a sealed pilot circuit of electroviscous fluid having pressure transmitting means for adjusting the control valve and electrically controlled valve means for varying electroviscous fluid pressure at the transmitting means; and a pumping means driveable by the primary fluid for circulating the electroviscous fluid within the sealed pilot circuit.
The control valve may be linearly or rotatably adjustable, linear movement being directly imparted to the valve by the pressure transmitting means and rotary movement being indirectly imparted via suitable torsional elements.
The electrically controlled valve means may comprise a known "Wheatstone Bridge" arrangement of four electroviscous valves disposed as series pairs in two parallel flow paths of the pilot circuit, two flexible-walled chambers comprising the pressure transmitting means, being respectively located intermediate each series pair. In this arrangement, the control valve preferably comprises a spool valve, the spool having two opposing end faces arranged in contact with the two flexible-walled chambers respectively.
The pumping means is preferably isolated from the electroviscous fluid to avoid the possibility of dirt entering the pilot circuit, such as may occur through the sliding seals of conventional in-circuit pumps. A suitable isolated pumping means includes: a fluidic oscillator located in a subsidiary flow line of the primary fluid for directing primary fluid to flow alternately through two parallel outlet flow lines; fluid pressure coupling means for communicating pressure of the primary fluid flowing through each of the two outlet flow lines respectively to the electroviscous fluid in the pilot circuit at two pumping stations thereof; and one-way flow means located in the pilot circuit intermediate the pumping stations.